Acoustic actuators most commonly act as point sources for producing sound, i.e., are used as speakers, but are also used for active noise and vibration control. The most common of these acoustic actuators or speakers are electromagnetic-based and electrostatic-based systems.
Electromagnetic actuators include permanent magnets and copper coils which can be relatively heavy and have relatively high profiles, even for low-power applications. The higher the spatial resolution desired from a speaker, the greater the number of electromagnetic actuators required. Accordingly, for applications requiring high spatial resolution but with weight and space limitations, such as in automotive and aerospace applications, electromagnetic acoustic actuators are impractical.
Electrostatic speakers are constructed with two electrode plates having different electrical potentials and positioned with a narrow air gap in between, with air being used as the dielectric medium. To produce sound, one of the plates is held stationary and the other is moved relative to the stationary plate. The movable plate is electrostatically attracted to the stationary plate. While electrostatic speakers are lightweight and can be made to have a relatively low profile, they have several disadvantages for many applications. These speakers tend to be costly since it is necessary to carefully construct the speaker so that the moving plate does not contact the stationary plate, but with a small enough air gap so that the driving voltage is not required to be excessive. Additionally, because the radiating plate must maintain a nearly constant spacing from a rigid stationary plate, these speakers are limited to flat-mounted applications. Further, as electrostatic speakers typically operate with a bias voltage of several thousand volts, limitations on the driving voltage will also limit the acoustic power output.
Speakers using piezoelectric ceramics and relatively rigid polymer materials as the dielectric layer are also known. With these speakers, sound is produced primarily by changing the thickness of the polymer film (or stack of films) due to the electrostrictive or piezoelectric effect. The polymer dielectric allows greater power output (per speaker surface area and weight) than air-gap-based electrostatic speakers at a given voltage. As the electrostatic energy is multiplied by the dielectric constant of the polymer, the polymer dielectric has a greater breakdown voltage than air in practical designs. Thus, since the applied voltage can be greater than that generated by air-gap devices, the electric field will also be greater, further increasing the power output capabilities of the actuator.
U.S. Pat. No. 6,343,129 discloses speakers using electroactive polymers having low moduli of elasticity in which the in-plane strains of the compliant electroactive polymer dielectric are used to induce out-of-plane deflection of die film to produce sound. The stiffness and mass of polymer films operating in this out-of-plane configuration are orders of magnitude less than that for compression of the more rigid polymers used in the electrostrictive and piezoelectric devices mentioned above. This allows for higher acoustic output per surface area and per weight at lower driving voltages than is possible with other electrostatic devices. Other advantages of speakers made with elastomeric polymer films is that they can be made in a wide variety of form factors, i.e., they can be conformed to any shape or surface, they are very lightweight and have very low-profiles that can be unobtrusively located on walls, ceilings or other surfaces, and they are relatively easy to manufacture and use low cost materials.
With the advantages provided to electrostatic speakers by use of dielectrics made of compliant electroactive polymer films, there is great interest in the improvement of speaker performance as well as other acoustic applications, such as active noise and vibration control systems, and non-acoustic applications, such as the control of airflow and turbulence on the surface of aircraft, ships, or other objects.